1. Field of the Invention
The present invention relates to an optical navigation device and a method for compensating for an offset in the optical navigation device and, more particularly, to an optical navigation device for compensating for an offset using the average of images of respective pixels and a method for compensating for the offset in the optical navigation device.
2. Description of Related Art
An optical navigation device successively acquires images reflected from a subject by allowing a light source to irradiate light to a subject (or the surface of a worktable), compares a current acquired image with a previously acquired image, and calculates a motion value based on the comparison result.
FIG. 1 is a diagram illustrating an image acquiring process of an optical mouse that is an example of a conventional optical navigation device.
Referring to FIG. 1, in a conventional optical mouse 1, light 7 is emitted from a light source 8 and reflected from a subject 2. When reflected light 6 is transmitted through a lens 5, an image sensor 3 including a semiconductor chip accumulates light 4, which is transmitted through the lens 5, and acquires the image of the subject 2. The image sensor 3 includes a plurality of pixels, each of which calculates the amount of the light 4 transmitted through the lens 5, compares the pixels with one another, and senses the image of the surface of the subject 2. The optical mouse 1 compares the current image of the subject 2 with the previously acquired image thereof and calculates a motion value of the optical mouse 1.
FIG. 2 is a block diagram of a conventional optical navigation device.
Referring to FIGS. 1 and 2, light 7 is emitted from a light source 10, reflected from a subject 2, and projected onto an image sensor 20. Thus, the image sensor 20 having a plurality of pixels accumulates the projected light 4 for a sampling period, acquires the image of the subject 2, generates an analog image signal An, and outputs the analog image signal An from each of the pixels. An analog-to-digital (A/D) converter 30 receives the analog image signal An from the image sensor 20, converts the analog image signal An into a digital image signal Xn, and outputs the digital image signal Xn.
An image filter 40 filters the digital image signal Xn and outputs a filtered image Fn. When the optical navigation device is embodied by an optical mouse, a data processing speed significantly affects the performance of the device. Therefore, the conventional optical navigation device includes image filtering means for reducing the amount of images to increase the data processing speed. The image filter 40 of FIG. 2 filters a digital image signal Xn having n-bit image quality in real-time and outputs a filtered image Fn having 1-bit (or sub-n-bit) image quality.
A motion value calculator 50 calculates a correlation between the previous filtered image acquired during the previous sampling period and the current filtered image Fn and outputs a motion value MV of the optical navigation device as a position of the current filtered image Fn that is most correlated with the previous filtered image.
There may be various offsets in the above-described optical navigation device. For example, there may be offsets in the image sensor 20. Although the image sensor 20 has a plurality of pixels, the respective pixels do not exhibit the same performance. The pixels of the image sensor 20 generate electric signals in response to incident light. In this process, the electric signals generated by the respective pixels may differ with respect to the same light intensity. A difference between the electric signals generated by the pixels corresponds to a difference in gain relative to light intensity between the pixels and results from the occurrence of offsets in a process.
Another cause of offsets in the optical navigation device may be lighting offsets that occur due to the non-uniformity of light emitted from the light source 10. In the optical navigation device, light emitted from the light source 10 is reflected from the subject 2 and projected onto the image sensor 20 through a lens. However, the light emitted from the light source 10 may not be applied to all surfaces of the subject 2 at the same light intensity, and the light reflected from the subject 2 also may not be projected onto the image sensor 20 at the same light intensity through the lens. Conventionally, one skilled in the art knows that the light intensity of an optical mouse has a tolerance of about 4%.
Furthermore, some offsets of the optical navigation device occur due to foreign materials. When there are foreign materials, such as dust, in a light incidence path of the optical navigation device, light cannot be projected onto the image sensor, and thus offsets can occur.
Moreover, long-term use of the optical navigation device leads to the occurrence of offsets. As the optical navigation device is used over a long period of time, the characteristics of the light source 10, the lens, and the image sensor 20 are changed.
In addition to the offsets, the image filter 40 also affects the operation of the optical navigation device. The image filter 40 reduces the amount of acquired images in order to increase the operating speed of the optical navigation device. In this process, the loss of images caused by the offsets may occur so that a motion value of the optical navigation device cannot be precisely calculated.
FIGS. 3A and 3B are diagrams illustrating a method of filtering an image using the image filter in the conventional optical navigation device show in FIG. 2.
In FIG. 3A, each of digital images signals X0, . . . , and X5 is a digital image signal Xn into which the A/D converter 30 converts an analog image signal An generated by the image sensor 20, for a predetermined period of time.
In FIG. 3B, each of filtering signals F0, . . . , and F5 is a filtering signal Fn that is obtained by filtering the corresponding one of the digital image signals X0, . . . , and X5 using the image filter 50.
Here, the digital image signals X0, . . . , and X5 and the filtering signals F0, . . . , and F5 are expressed as digital image matrices and filtering matrices, respectively, for brevity.
Referring to FIG. 3A, the digital image matrices X0, . . . , and X5 are arranged in 7 rows and 7 columns. Also, the digital image matrix X0 is an initial digital image matrix in which a right column of the matrix is brighter than a left column thereof due to offsets caused by the above-described offsets of the image sensor 20, such as lighting offsets or foreign materials.
As the optical navigation device moves to the left, a predetermined pattern is formed from the left column to the right column in the digital image matrices X1, . . . , and X5. Specifically, a predetermined pattern is formed in a first column in the digital image matrix X1, a predetermined pattern is formed in a second column in the digital image matrix X2, and a predetermined pattern is formed in a fifth column in the digital image matrix X5.
The image filter 40 may be of various kinds. Here, an image filter of FIG. 3B outputs the filtering matrix F0 . . . , and F5 having 1-bit image quality as the filtering signal Fn. That is, the image filter of FIG. 3B outputs “1” when a value obtained by subtracting a value of a left cell from a value of each cell of the digital image matrix X0, . . . , and X5 is greater than “0”, and outputs “0” when the obtained value is less than or equal to “0”.
For example, a value “1” of a cell (3, 3) of the filtering matrix F0 is found since a value “2”, which is obtained by subtracting a value “5” of a cell (3, 3) of the digital image matrix X0 from a value “7” of a cell (3, 4) of the digital image matrix X0, is greater than “0”.
However, as shown in the filtering matrix F0, . . . , and F5, when images are filtered using the above-described image filter 40 in the optical navigation device having offsets, a value of every cell of the filtering matrix F0, . . . , and F5 turns out to be “1” so that the optical navigation device cannot calculate the motion value MV.
It is exemplarily described above with reference to FIGS. 3A and 3B that the image filter 40 filters images by subtracting a value of the left cell from a value of each cell of the digital image matrix X0, . . . , and X5. However, even if other kinds of image filters are used, malfunctions may occur in the conventional optical navigation device owing to various offsets.